Bug in PlanarImg accessors

[tpietzsch]

There is most likely a few bugs in PlanarImg accessors.

For example:
PlanarCursor has a protected int index field that is used and modified in jumpFwd() and fwd() among others.
PlanarCursor2D extends PlanarCursor and overrides fwd() such that it does not touch index. So, in subsequent jumpFwd() it would be out-of-sync with the Type's index.

There is a few other instances of similar problems in the net.imglib2.img.planar. I didn't thoroughly investigate, write tests, etc. But something is clearly wrong.